Winding corrugated and flat thin metal foils together in a cylindrical package for use in rotating heat exchangers, exhaust gas purifiers or sound dampers is previously known. A plurality of longitudinal ducts will be formed between the corrugated and the flat foil, allowing a stream of gas or liquid to flow through the ducts. These applications have the common feature of aiming at achieving a large contact area between the foil and the flow, with the front surface limited. In addition, it is desirable to keep the pressure drop over the foil body low, partly in order to reduce the need for pump action, and partly to avoid damages that might break the foil package. Conventional techniques for retaining a foil package are point welding, soldering or transverse folds, as described in EP 604,868, U.S. Pat. No. 4,719,680 and WO93/02792.
The foil package is usually equipped with various layer coatings, for instance active layers of platinum metal with carriers in exhaust gas purifiers, or hygroscopic layers in heat exchangers. In this conjunction, another aim is to be able to add such layers with as even a thickness as possible, and without agglomerations at the duct angles, since locally thicker layers restrict the flow-through area and entail unnecessary consumption of layer material, which often is expensive.
In conventional applications, the foil folds have a relatively great radius or contact surface with the flat foil, and in that case the flow will not contact these surfaces. The purpose of the present invention is to provide a corrugated metal foil so as to increase the flow-through area, reduce the flow resistance and cut the material consumption for layer coating.
In accordance with the present invention, a method for corrugating a metal foil is disclosed, in which an originally flat metal foil is rolled in at least two steps between fluted rollers disposed in pairs. In a first step, the roller grooves have a radius at their top which accounts for 10% or more of the distance between the groove tops. In a final step, the roller grooves have a radius at their top which is smaller than the radius in the first step.